1. Technical Field
The present invention generally relates to carbon materials comprising an optimized pore structure, methods for making the same and devices containing the same.
2. Description of the Related Art
Activated carbon is commonly employed in electrical storage and distribution devices. The high surface area, conductivity and porosity of activated carbon allows for the design of electrical devices having higher energy density than devices employing other materials. Electric double-layer capacitors (EDLCs or “ultracapacitors”) are an example of such devices. EDLCs often have electrodes prepared from an activated carbon material and a suitable electrolyte, and have an extremely high energy density compared to more common capacitors. Typical uses for EDLCs include energy storage and distribution in devices requiring short bursts of power for data transmissions, or peak-power functions such as wireless modems, mobile phones, digital cameras and other hand-held electronic devices. EDLCs are also commonly use in electric vehicles such as electric cars, trains, buses and the like.
Batteries are another common energy storage and distribution device which often contain an activated carbon material (e.g., as anode material, current collector, or conductivity enhancer). For example, lithium/carbon batteries having a carbonaceous anode intercalated with lithium represent a promising energy storage device. Other types of carbon-containing batteries include lithium air batteries, which use porous carbon as the current collector for the air electrode, and lead acid batteries which often include carbon additives in either the anode or cathode. Batteries are employed in any number of electronic devices requiring low current density electrical power (as compared to an EDLC's high current density).
One known limitation of EDLCs and carbon-based batteries is decreased performance at high-temperature, high voltage operation, repeated charge/discharge cycles and/or upon aging. This decreased performance has been attributed, at least in part, to electrolyte impurity or impurities in the carbon electrode itself, causing breakdown of the electrode at the electrolyte/electrode interface. Thus, it has been suggested that EDLCs and/or batteries comprising electrodes prepared from higher purity carbon materials could be operated at higher voltages and for longer periods of time at higher temperatures than existing devices.
In addition to purity, another known limitation of carbon-containing electrical devices is the pore structure of the activated carbon itself. While activated carbon materials typically comprise high porosity, the pore size distribution is not optimized for use in electrical energy storage and distribution devices. Such optimization includes an idealized blend of both micropores and mesopores. An idealized pore size distribution is expected to maximize ion mobility (i.e., lower resistance), increase power density and improve volumetric capacitance of electrodes prepared from the optimized carbon materials.
Although the need for improved high purity carbon materials comprising a pore structure optimized for high pulse power electrochemical applications has been recognized, such carbon materials are not commercially available and no reported preparation method is capable of yielding the same. One common method for producing high surface area activated carbon materials is to pyrolyze an existing carbon-containing material (e.g., coconut fibers or tire rubber). This results in a char with relatively low surface area which can subsequently be over-activated to produce a material with the surface area and porosity necessary for the desired application. Such an approach is inherently limited by the existing structure of the precursor material, and typically results in a carbon material having an unoptimized pore structure and an ash content (e.g., metal impurities) of 1% or higher.
Activated carbon materials can also be prepared by chemical activation. For example, treatment of a carbon-containing material with an acid, base or salt (e.g., phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, etc.) followed by heating results in an activated carbon material. However, such chemical activation also produces an activated carbon material not suitable for use in high performance electrical devices.
Another approach for producing high surface area activated carbon materials is to prepare a synthetic polymer from carbon-containing organic building blocks (e.g., a polymer gel). As with the existing organic materials, the synthetically prepared polymers are pyrolyzed and activated to produce an activated carbon material. In contrast to the traditional approach described above, the intrinsic porosity of the synthetically prepared polymer results in higher process yields because less material is lost during the activation step. However, known methods for preparing carbon materials from synthetic polymers produce carbon materials having unoptimized pore structures and unsuitable levels of impurities. Accordingly, electrodes prepared from these materials demonstrate unsuitable electrochemical properties.
While significant advances have been made in the field, there continues to be a need in the art for improved high purity carbon materials comprising an optimized pore structure for use in electrical energy storage devices, as well as for methods of making the same and devices containing the same. The present invention fulfills these needs and provides further related advantages.